Structure and Stability of a Dimeric G-Quadruplex Formed by Cyclic Oligonucleotides

We have studied the structure and stability of the cyclic dodecamer d<pGGGTTAGGGTTA>, containing two copies of the human telomeric repeat. In the presence of sodium, NMR data are consistent with a dimeric structure of the molecule in which two cycles self-associate forming a quadruplex with three guanine tetrads connected by edgewise loops. The two macrocycles are arranged in a parallel way, and the dimeric structure exhibits a high melting temperature. These results indicate that cyclization of the phosphodiester chain does not prevent quadruplex formation, although it affects the global topology of the quadruplex

The Oxidative Coupling of 2,6-Xylenol Catalyzed by Polymeric Complexes of Copper, 2. Physicochemical Study on Copper(II) Complexes of Partially Dimethylaminomethylated Polystyrene

The polymeric catalyst formed by complexation of copper(II) chloride and partially dimethylaminomethylated polystyrene was investigated to explain its behaviour in the oxidative coupling of 2,6-xylenol. Viscometric studies indicated that at low polymer concentrations coordination of tertiary amine groups to copper(II) causes an intramolecular crosslinking. UV measurements and preliminary results of ESR point to a dimeric structure of these complexes with two amine groups per copper. A mechanism for the action of this polymeric catalyst is suggested, based on these results and on those described in Part 1. It appeared that some "free" copper(II) is essential for the catalytic activity, without which the reoxidation of copper(I) cannot take place.

Frustrated Spin System in theta-(BEDT-TTF)_2RbZn(SCN)_4

The origin of the spin gap behavior in the low-temperature dimerized phase of theta-(BEDT-TTF)_2RbZn(SCN)_4 has been theoretically studied based on the Hartree-Fock approximation for the on-site Coulomb interaction at absolute zero. Calculations show that, in the parameter region considered to be relevant to this compound, antiferromagnetic ordering is stabilized between dimers consisting of pairs of molecules coupled with the largest transfer integral. Based on this result an effective localized spin 1/2 model is constructed which indicates the existence of the frustration among spins. This frustration may result in the formation of spin gap.Comment: 4 pages, 5 figures, to be published in J. Phys. Soc. Jpn. 67 (1998) no.

Lithium and aluminium carbamato derivatives of the utility amide 2, 2, 6, 6- tetramethylpiperidide

Insertion of CO2 into the metal-N bond of a series of synthetically-important alkali-metal TMP (2,2,6,6-tetramethylpiperidide) complexes has been studied. Determined by X-ray crystallography, the molecular structure of the TMEDA-solvated Li derivative shows a central 8-membered (LiOCO)2 ring lying in a chair conformation with distorted tetrahedral lithium centres. While trying to obtain crystals of a THF solvated derivative, a mixed carbonato/carbamato dodecanuclear lithium cluster was formed containing two central (CO3)2- fragments and eight O2CTMP ligands with four distinct bonding modes. A bisalkylaluminium carbamato complex has also been prepared via two different methods (CO2 insertion into a pre-formed Al-N bond and ligand transfer from the corresponding lithium reagent) which adopts a dimeric structure in the solid state

Crystal Structures of Influenza A Virus Matrix Protein M1: Variations on a Theme

Matrix protein 1 (M1) of the influenza A virus plays multiple roles in virion assembly and infection. Interest in the pH dependence of M1\u27s multiple functions led us to study the effect of subtle pH changes on M1 structure, resulting in the elucidation of a unique low-pH crystal structure of the N1-165-domain of A/WSN/33 (H1N1) M1 that has never been reported. Although the 2.2 Å crystal structure of M1 N-terminus shows a dimer with the two monomers interacting in a face-to-face fashion at low pH as observed earlier, a 44° rotation of the second monomer has led to a significantly different dimer interface that possibly affects dimer stability. More importantly, while one of the monomers is fully defined, the N-terminal half of the second monomer shows considerable disorder that appears inherent in the protein and is potentially physiologically relevant. Such disorder has not been observed in any other previously reported structure at either low or high pH conditions, despite similar crystallization pH conditions. By comparing our novel N1-165-domain structure with other low-pH or neutral-pH M1 structures, it appears that M1 can energetically access different monomer and dimer conformations, as well as oligomeric states, with varying degree of similarities. The study reported here provides further insights into M1 oligomerization that may be essential for viral propagation and infectivity

Synthesis and Structure of 1,2-Dimethylene[2.10]metacyclophane and Its Conversion into Chiral [10]Benzenometacyclophanes: Synthesis and Structure of 1,2-Dimethylene[2.10]metacyclophane and Its Conversion into Chiral [10]Benzenometacyclophanes

Bromination of 5,21-di-tert-butyl-8,24-dimethoxy-1,2-dimethyl[2.10]metacyclophan-1-ene (MCP-1-ene; 1) with benzyltrimethylammonium tribromide exclusively afforded 1,2-bis(bromomethyl)-5,21-di-tert-butyl-8,24-dimethoxy[2.10]MCP-1-ene (2). Debromination of 2 with Zn and AcOH in CH₂Cl solution at room temperature for 24 h produced dimethylene[2.10]MCP 7 in 92 % yield, which is a stable solid compound. Compound 7 was treated with dimethyl acetylenedicarboxylate (DMAD) to provide 1,2-(3′,6′-dihydrobenzo)-5,21-di-tert-butyl-8,24-dimethoxy[2.10]MCP-4′,5′-dimethylcarboxylate (8) in good yield. Diels–Alder adduct 8 was converted into a novel and inherently chiral areno-bridged dimethoxy[2.10]MCP-4′,5′-dimethylcarboxylate 9, possessing C₁ symmetry, by aromatization with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). A new type of N-phenyl-maleimide substituted 1,2-(3′,6′-dihydrobenzo)-5,21-di-tert-butyl-8,24-dimethoxy[2.10]MCP-4′,5′-N-phenylmaleimide 10 was also synthesized from 7 through treatment with N-phenylmaleimide in toluene at 110 °C followed by aromatization with DDQ. Single-crystal X-ray analysis of 9 revealed the formation of a syn-isomer

A homoleptic phosphine adduct of Tl(I)

A homoleptic phosphine adduct of thallium(I) supported by a tris(phosphino)borate ligand has been isolated and structurally characterized

Di-μ-chlorido-bis­{[2-(morpholinometh­yl)phenyl-κ2 C 1,N]palladium(II)}

The title compound, [Pd2(C11H14NO)2Cl2], has a dimeric structure with Cl atoms bridging the two Pd atoms, one half of the mol­ecule being generated by symmetry due to the crystallographic inversion centre located in the middle of the perfectly planar Pd2Cl2 ring. The five-membered ring adopts an envelope conformation, while the morpholino group has a chair conformation. The geometry around the metal centres is distorted square-planar, as a result of a strong intra­molecular N→Pd coordination trans to a Pd—Cl bond. In the crystal structure, the dimeric structure is strengthened by inter­molecular C—H⋯Cl hydrogen bonds. C—H⋯Cphen­yl inter­actions link the dimers into a columnar supra­molecular array along the a axis; the dimers are further connected by C—H⋯Ph inter­actions into a three-dimensional supra­molecular arrangement

Spectroscopic characterization of reaction centers of the (M)Y210W mutant of the photosynthetic bacterium Rhodobacter sphaeroides

The tyrosine-(M)210 of the reaction center of Rhodobacter sphaeroides 2.4.1 has been changed to a tryptophan using site-directed mutagenesis. The reaction center of this mutant has been characterized by low-temperature absorption and fluorescence spectroscopy, time-resolved sub-picosecond spectroscopy, and magnetic resonance spectroscopy. The charge separation process showed bi-exponential kinetics at room temperature, with a main time constant of 36 ps and an additional fast time constant of 5.1 ps. Temperature dependent fluorescence measurements predict that the lifetime of P* becomes 4–5 times slower at cryogenic temperatures. From EPR and absorbance-detected magnetic resonance (ADMR, LD-ADMR) we conclude that the dimeric structure of P is not significantly changed upon mutation. In contrast, the interaction of the accessory bacteriochlorophyll BA with its environment appears to be altered, possibly because of a change in its position